1. Field of the Invention
The present invention relates to the magnetic resonance imaging technology field, and more particularly to a dynamic frequency drift correction method for binomial water excitation method.
2. Description of the Prior Art
In magnetic resonance imaging (MRI), the body's molecular environment for hydrogen protons in fat tissue is different from that for hydrogen protons in other tissues, which results in a difference in resonance frequency of the protons; the relaxation time of the hydrogen protons in fat tissue and those in other tissues is also different when they are excited by the radio frequency pulses at the same time. When collecting signals at different echo times, the fat tissue and non-fat tissues show different phases and signal strengths.
In magnetic resonance imaging field, the methods commonly used to inhibit fat signals include fat saturation, inversion recovery, water-fat separation, etc. In the fat saturation method, the MRI device selects a radio frequency (RF) pulse at a certain frequency, and uses the RF pulse to invert the fat signals to the transverse plane, and then applies a spoiler gradient to eliminate the fat signals prior to the imaging RF pulse. In the inversion recovery method, the MRI device uses a 180° pulse to invert all signals to the negative polar axis, and applies the imaging RF pulse when the fat signal returns to zero in the longitudinal axis after the relaxation time (TI) to obtain the images with no fat signal. In the water-fat separation method such as the Dixon method, the MRI device uses multiple images with different echo times (TE) to perform water-fat separation calculation.
The fat saturation method can be used in the spin echo sequence and gradient echo sequence, but it cannot be used in a low magnetic field MRI system, because there is a little frequency offset caused by chemical shift in such a low magnetic field MRI system. The inversion recovery method can be widely used in various sequences and the signal-to-noise ratio (SNR) of the images produced by this method is low, but its scanning is time-consuming.
The binomial water excitation method is a compromise of the fat inhibition method, and can be used in various sequences for both low field MRI system and high field MRI system. Compared to the fat saturation method, the binomial water excitation method has a similar SNR but the scanning time is reduced remarkably. The main problem of the binomial water excitation method is its sensitiveness to the stability of the main (basic) magnetic field (B0), which limits the use of the binomial water excitation method in the permanent magnet MRI system.
For the binomial water excitation method, the frequency scout technology can be used to find the optimal static frequency offset to compensate inhomogeneity in the magnetic field for better inhibition of the fat. However, frequency scout cannot solve the frequency drift problem of the main magnetic field mainly caused by temperature rise in the magnet material.
Chinese patent application No. 200510068442.0, “A Method for Dynamic Detection of Resonance Frequency in Magnetic Resonance Spectroscopy Tests”, a method used to perform dynamic frequency detection of magnetic resonance frequency in the magnetic resonance spectroscopy tests. The method includes: measuring the navigation signals respectively at the same time in each sequence flow of multiple sequence flows which are performed in turn, and determining the frequency drift of the magnetic resonance frequency by comparing these navigation signals, and then using the measured frequency drift to correct the individual frequency spectrums obtained from each of the sequence flows.